1. Field of the Invention
This invention relates in general to memory paging techniques, and relates more particularly to a mechanism for extending in a non-linear fashion a given amount of address space, without modifying the memory map as seen by the host central processing unit (CPU).
2. Prior Art
The present addressing technique may be applied to extending from 32KBytes to 64KBytes the existing address space reserved by the IBM PC/AT and IBM PC/XT for the Video Basic Input Output System (BIOS). This technique preserves the original CPU address map, so that existing programs can still run and will see exactly the same address space.
Preserving the CPU address space to 32KBytes is an IBM PC/AT and PC/XT compatibility requirement. The memory map of these systems is allocated among various locations and the Video BIOS must be stored within a specific, standardized address range. Departing from the 32KBytes allocated to the Video BIOS would mean a likely addressing conflict with other non-Video parts of the operating system or with applications programs which use that addressing space reserved for them by the standard IBM PC-AT and PC/XT memory map.
At the same time, Video subsystems are becoming more complex and requiring a larger BIOS to support them. If the effective address space available to the Video BIOS is not extended from 32KBytes to 64KBytes, it would become a major impediment to Video display capabilities.